first_of¶
Header¶
#include <boost/hof/first_of.hpp>
Description¶
The first_of
function adaptor combines several functions together. If
the first function can not be called, then it will try to call the next
function. This can be very useful when overloading functions using
template constraints(such as with enable_if
).
Note: This is different than the match
function adaptor, which
can lead to ambiguities. Instead, first_of
will call the first function
that is callable, regardless if there is another function that could be
called as well.
Synopsis¶
template<class... Fs>
constexpr first_of_adaptor<Fs...> first_of(Fs... fs);
Example¶
#include <boost/hof.hpp>
#include <iostream>
using namespace boost::hof;
struct for_ints
{
void operator()(int) const
{
printf("Int\n");
}
};
struct for_floats
{
void operator()(float) const
{
printf("Float\n");
}
};
int main() {
first_of(for_ints(), for_floats())(3.0);
}
This will print Int
because the for_floats
function object won’t ever be
called. Due to the conversion rules in C++, the for_ints
function can be
called on floats, so it is chosen by first_of
first, even though
for_floats
is a better match.
So, the order of the functions in the first_of_adaptor
are very important
to how the function is chosen.